For most optoelectronic products manufactured today, coupling light into or out of an optical device is accomplished by active alignment of one device with respect to another. The basic process is to move an object in space angularly and laterally to locate a first device [X,Y,Z] and orient [θx, θy, θz] it with respect to a second device. The device can be held either by mechanical clamp or suction generated by vacuum pump. Special toolings are usually made for particular geometry.
To maintain alignment, the first device has to be permanently fixed on a motherboard. The challenge is to find a suitable mounting technique that will allow sufficient angular and lateral offset as the fixture secured to a motherboard. There are usually arbitrary gaps formed between bonding surfaces of the optical device and the motherboard, as depicted in FIG. 1 of prior art, due to physical impression of parts. In FIG. 1, a first optical device is aligned respect to a second optical device to couple the light into or out of these optical devices. The gap between surface 1 of the first device and surface 2 of the motherboard is formed. These gaps inhibit the aligned assembly from being assembled with solid contacts.
FIGS. 2, 3, and 4 demonstrate various prior art assembly concepts to compensate for such angular and lateral deviations. Typical solutions involve the use of thick epoxy and/or solder and precision spacers. FIG. 2 shows the gap between two bonding surfaces is filled with epoxy. The problem with this approach is that epoxy shrinks during curing. The resulting dislocation could be significant if the gap is large. This shrinkage is generally predictable and could be accounted for in final assembly. However, this can make the assembly process complicated and often unreliable. FIG. 3 depicts enhanced approach that uses a spacer to reduce the overall gap between the optical device and the motherboard. A layer of epoxy fills the subgap between the optical device and the spacer. The thickness of the spacer has to be precise to properly align the first device with respect to the second. Furthermore, shrinkage of the epoxy during curing is still a problem. Another approach, shown in FIG. 4, is to use a solder bump, allowing two surfaces to be bonded with solder reflow at high temperature. Although many advantages of this technology have been realized: high yield, high strength and self-alignment during joining, the initial setup cost is extremely high. Furthermore, the device is not secured to the motherboard during solder reflow and may become misaligned as the solder solidifies. In addition, the solder bump may not be able to withstand large temperature fluctuations due to differences in the coefficients of thermal expansion of the bonding materials. The problem becomes aggravated as the size of solder becomes larger.
There is a need, therefore, for a low cost packaging method to assemble pre-aligned optical modules to a common substrate, by which the optical modules are permanently fixed on the common substrate without dislocation due to temperature variations.